That's a truly impressive feat since Apple just launched the iPad two and a half years ago. But, as impressive as it is, it's going to be a big let down for investors when Apple reports earnings on Thursday.

Analysts are expecting Apple to announce 17.6 million iPads sold for the quarter, according to Fortune.

But, if Apple sold its 100 millionth iPad two weeks ago, then it means Apple sold less than 16 million last quarter.

We used Gene Munster's models on Apple's business to add up how many iPads Apple has sold. According to Munster's spread sheet, Apple sold 84.1 million iPads from the time it launched to the end of the the second calendar quarter of this year. Subtract that from 100 million iPads total and you get 15.9 million iPads.

And that's skipping the first week of October. Therefore, Apple probably sold less than 15.9 million iPads last quarter. Since Apple is selling ~1 million iPads per week, it probably sold just 15 million iPads last quarter.